Conventional resulfurized free-machining austenitic stainless steels such as AISI Type 303 generally do not have sufficient corrosion resistance to allow them to be used in applications for acid soft drink or beverage syrups without significantly affecting the flavor of these products. The problem largely relates to the fact that the manganese or manganese-rich sulfides present in Type 303 are readily attacked in acid soft drink or beverage syrups. As a result of this attack, the local environment is so changed that the stainless steel adjacent to the manganese or manganese-rich sulfides corrodes, thereby releasing both sulfide and metal ions into the syrups and causing odor or taste problems. Passivating free-machining austenitic stainless steels such as AISI Type 303 in nitric acid solutions can minimize this difficulty by removing most of the manganese or manganese-rich sulfides from the surfaces of the articles machined from these steels before they are placed in service. However, the general corrosion resistance of the stainless steel matrix of AISI Type 303 is often insufficient, even in the absence of substantial sulfide dissolution, to avoid changes in the quality or taste of the beverage syrups. Thus, to improve the corrosion resistance of free-machining austenitic stainless steels in acid beverage syrups, the use of a more corrosion resistant free-machining additive along with improvements in the general corrosion resistance of the steel matrix in acid beverage syrups are necessary.
In this respect, U.S. Pat. No. 3,902,398 discloses that the corrosion resistance of resulfurized free-machining austenitic stainless steels can be significantly improved in acid beverage syrups by restricting their manganese content to a maximum of about 0.50% and by controlling the manganese to sulfur ratio such that chromium or chromium-rich sulfides are formed instead of manganese or manganese-rich sulfides. Chromium sulfides are more corrosion resistant than are manganese or manganese-rich sulfides in acid beverage syrups, and improve machinability but not nearly to the same extent as manganese or manganese-rich sulfides. As also disclosed in U.S. Patent 3,902,398, the loss in machinability related to the replacement of manganese or manganese-rich sulfides by chromium sulfides can be partly offset by lowering the carbon content of such steels to below about 0.035%.
In accordance with the present invention, the machinability of low-carbon resulfurized austenitic stainless steels containing chromium or chromium-rich sulfides can be substantially improved by controlling their carbon plus nitrogen content to lower than conventional levels. It has further been discovered that the addition of copper, which is known to improve the machinability of other austenitic stainless steels, not only improves the machinability of these low-manganese free-machining austenitic stainless steels, but also significantly improves their corrosion resistance in acid soft drink and beverage syrups. Thus, it is possible to substantially improve the machinability of free-machining austenitic stainless steels containing chromium or chromium-rich sulfides by lowering their carbon plus nitrogen content below conventional levels, and to further improve their machinability and especially their resistance to corrosion in acid soft drink or beverage syrups by increasing their copper content within closely controlled limits.